This invention relates to an apparatus and a method enabling the continuous feeding of alumina to an electrolytic cell for the production of aluminium by the Hall-Heroult process. In this process, aluminium is produced by electrolytic reduction of alumina (Al.sub.2 O.sub.3) dissolved in a bath of electrolyte contained in the cell which electrolyte is based on molten cryolite. The metal is formed at the molten aluminium cathode and oxygen is discharged at the carbon anode but reacts with and consumes it to form carbon oxides.
An important factor in the efficient production of aluminium by this process is the method whereby the alumina is introduced into the bath. A method using traditional technology involves breaking the frozen crust alumina matrix which covers the surface of the molten electrolyte in the cell in normal operation to form an opening and then dumping a volumetrically measured amount of alumina into the opening. This dumping is done either at periodic intervals or on receipt of a demand feed signal. By way of example, one approach is to break all the crust along half the centre channel width of the cell and dump the volumetric equivalent of approximately 65 kg of alumina into the cell every 30-100 minutes or on a demand feed basis. This causes a major thermal disturbance as the alumina concentration of the electrolyte increases and subsequently decreases as it is electrolytically removed.
Recent experiments with the composition of the electrolyte and the operation of electrolytic cells have indicated that the crust added during the breaking action contributes to operating problems as does the mass of frozen electrolyte alumina matrix that is generated by the thermal shock of a dumping. Consequently, smaller alumina additions have become favoured to reduce the sludging of the alumina and thus increase the efficiency of the system. This has led to the development of point feeding systems where a smaller hole is broken through the crust and the mass of alumina added during each feeding action varies between 0.5 and 3 kg, but the feeding frequency is increased to every 1 to 5 minutes depending on the feed strategy used. Because of the more frequent feeding of smaller quantities of alumina, more than one hole is formed in the crust by pneumatically or mechanically driven pickles (crust breakers). Examples of point or spot feeding systems are given in U.S. Pat. Nos. 3,400,062, 3,689,229 and 4,437,964.
U.S. Pat. No. 4,431,491, assigned to Pechiney, discloses a point feeder for feeding alumina to an electrolytic cell. The apparatus includes a distributor and a piercing tool. The distributor comprises a metering means which contains a fixed quantity of alumina. In operation, the lower part of the metering means is moved downwardly to form an opening through which the fixed quantity of alumina is dumped into a hole in the crust on the cell.
Other point feeders generally incorporate a dosage container which can store a known (and fixed) amount of alumina. The dosage container is periodically operated to dump the dose of alumina into the cell. The dosage container is generally operated in synchronicity with the crust breaking means.
While the point feeding systems have been a bag improvement on the larger dump feeding system, the discontinuous addition of discrete quantities of alumina to the cell still causes inefficient alumina dissolution, a limited amount of sludge formation, and fluctuations in the concentration of alumina in the electrolyte. Modern understanding indicates the desirability of preventing any sludge build up and minimising concentration fluctuations, especially where it is desired to operate the tank with an electrolyte near its freezing point or near its alumina saturation point. It was with this in mind that the apparatus and method of the present invention were devised.